An axial flow, gas turbine engine typically has a compression section, a combustion section, and a turbine section. An annular flow path for working medium gases extends axially through these sections of the engine. A stator assembly extends about the annular flow path for confining the working medium gases to the flow path and for directing the working medium gases along the flow path.
As the gases are passed along the flow path, the gases are pressurized in the compression section and burned with fuel in the combustion section to add energy to the gases. The hot, pressurized gases are expanded through the turbine section to produce useful work. A major portion of this work is used as output power, such as for driving a free turbine or developing thrust for an aircraft.
A remaining portion of the work generated by the turbine section is not used for output power. Instead, this portion of the work is used in the compression section of the engine to compress the working medium gases. The engine is provided with a rotor assembly for transferring this work from the turbine section to the compression section. The rotor assembly has arrays of rotor blades in the turbine section for receiving work from the working medium gases. The stator assembly has arrays of stator vanes which extend inwardly across the working medium flow path between the arrays of rotor blades. The stator vanes direct the approaching flow to the rotor blades at a desired angle. The rotor blades have airfoils that extend outwardly across the working medium flow path and that are angled with respect to the approaching flow to receive work from the gases and to drive the rotor assembly about the axis of rotation.
The stator assembly further includes an outer case and arrays of wall segments supported from the outer case which extend circumferentially about the working medium flow path. The wall segments are located adjacent to the working medium flow path for confining the working medium gases to the flow path. These wall segments have radial faces which are circumferentially spaced leaving a clearance gap G therebetween. The clearance gap is provided to accommodate changes in diameter of the array of wall segments in response to operative conditions of the engine as the outer case is heated and expands or is cooled and contracts.
One example of an array of wall segments is the array of stator vanes. Each wall segment of the array of stator vanes bounds the working medium flow path and has one or more of the airfoils which extend inwardly across the working medium flow path. Another example of an array of wall segments is an outer air seal formed of circumferentially adjacent wall segments which extend about an array of rotor blades in close proximity to the airfoils for confining the working medium gases to the flow path.
The wall segments of the outer air seal and stator vanes are in intimate contact with the hot working medium gases and receive heat from the gases. The segments are cooled to keep the temperature of the segments within acceptable limits. One example of such a coolable array of wall segments is shown in U.S. Pat. No. 3,583,824 issued to Smuland et al. entitled "Temperature Controlled Shroud and Shroud Support". Smuland employs an outer air seal which is disposed outwardly of an array of rotor blades. Cooling air is flowed along a primary flow path in a cavity which extends circumferentially about the outer air seal between the outer air seal and the engine case. The cooling air is flowed through an impingement plate to precisely meter and direct the flow of cooling air against the outer surface of the wall segment. The air is gathered in an impingement air cavity and exhausted from the impingement air cavity into the working medium flow path to provide a continuous flow of fluid through the plate and against the wall segment. This cooling air provides convective cooling to the edge region of the outer air seal and to the adjacent structure as it passes through the outer air seal into the working medium flow path.
A seal member is typically provided in modern engines between each pair of circumferentially spaced wall segments. The seal member bridges the gap G between the segments to block the leakage of the cooling air between the segments into the working medium flow path. One example of wall segments showing this feature is in U.S. Pat. No. 3,341,172 issued to Rahaim entitled "Fluid Machine Casing Seal Structure". Rahaim discloses a C-shaped seal member extending between blocks 55b, as shown in FIG. 3 and FIG. 6, to prevent the leakage of cooling air from the exterior of the engine into the working medium flow path.
Another example of an array of wall segments provided with seal members is shown in U.S. Pat. No. 3,752,598 issued to Bowers et al. entitled "Segmented Duct Seal". Bowers et al. shows an array of stator vanes in FIG. 1 and FIG. 2 having circumferentially extending seal members extending between adjacent stator vanes. A primary flow path for cooling air, such as the flow paths 36 and 38, supplies cooling air to the interior of the vanes through openings in the ends of the vanes. The seal member is a seal plate 50 disposed in facing grooves between adjacent segments. The seal plate bridges the gap between the segments to block the leakage of cooling air along a leak path between the segments which extends from the primary flow path to the working medium flow path. These seal plates, though effective in blocking the leakage of working medium gases along the flow path, do not form a leak proof seal. This leakage is acceptable because it provides cooling to portions of the wall segments which are adjacent to the gap G and which are heated by the working medium gases on both a radial face and a circumferential face.
Although the use of cooling air is accepted because it increases the service life of wall segments and airfoils in comparison to uncooled wall segments and uncooled airfoils, the use of cooling air decreases the operating efficiency of the engine. This decrease occurs because a portion of the engine's useful work is used to pressurize the cooling air in the compression section decreasing the amount of useful work available for output power. One way to increase operating efficiency is to decrease the leakage of cooling air from the cooling air flow paths in the engine. Another way to increase operating efficiency is to more effectively use the cooling air so that increased cooling is provided with the same amount of cooling air or so that the same amount of cooling is provided with a decreased amount of cooling air.
Accordingly, scientists and engineers are seeking to more efficiently supply cooling air to components, such as the wall segments, by both improving the sealing structure and by more effectively using the cooling air supplied to the components.